Induction of antibody formation is directly related to cellular and molecular events, including cell recruitment, differentiation and proliferation. This study will extend previous investigations in this laboratory concerning cell populations involved in antibody formation utilizing a new immunoplaque assay permitting detailed analysis of antibody formation in the absence of pre-existing "natural" antibody. Our method is based on the classic Pfeiffer phenomenon of vibriolysis by specific antibody and complement. We have adapted this lytic reaction, using viable bacteria as the indicator, to the now classic Jerne plaque assay. No "background" antibody-forming cells to vibrio are present in the spleen or other lymphoid organs of various animal species tested before active immunization. Thus the kinetics and appearance of specific vibriolytic immunocytes are unique since no plaque forming cells are present until two to three days after immunization. This lag period is consistent. Furthermore, the increase in the number of immunocytes to vibrio during the early phase of the immune response is "stepwise," with a doubling time as short as two to three hours. As an additional feature, several distinct and characterizable cell wall antigens are present on individual bacterial cells. The immune response to each antigen can be separately enumerated by our plaque assay, permitting analysis of the cellular immune response to different antigens present on a single bacterium. These features will permit us to determine precisely: (1) the true latent period of antibody formation; (2) the number of initial antibody-forming cells appearing after differentiation; (3) the site of early accumulation of early antibody forming cells; (4) and molecular events occurring during the lag period, including new "species" of RNA. These parameters cannot be readily studied by other systems due to "masking" effects of the natural background antibody on the early immune response. The parameters to be studied are widely recognized as distinctive features of immune differentiation of immunocytes and are related to effect of antigen dose, the number of antigen-reactive cells, the number of stimulated cells, and the presence or absence of carrier-antigen specific cells. It is anticipated that use of our model will permit a definitive characterization of the early cellular events during immunogenesis.